The present invention relates in general to polyolefins, and more specifically to additive packages for polyolefins. The polyolefins made with these additive packages can be used to form articles which will exhibit an enhanced stain and discoloration resistance upon exposure to high temperature and harsh chemical conditions.
Pressurized automobile engine radiator overflow bottles, known as surge tanks, have been molded from polyolefins such as polypropylene for about the past ten to twelve years. These surge tanks were designed for use with traditional (inorganic) engine coolants, such as Prestone(copyright) and to exposure to reasonable pressures and temperatures. The general composition of Prestone(copyright) fluid, according to the manufacturer is ethylene glycol, diethylene glycol and water. Newer engine coolants such as Valvoline(copyright) G0-5 and Texaco(copyright) DEX-COOL(copyright) have been developed for longer use. The general composition of G0-5 coolant is ethylene glycol, diethylene glycol, and sodium tetraborate pentahydrate. According to its manufacturer, DEX-COOL(copyright) coolant contains 1,2-ethanediol, 2-ethyl hexanoic acid, and potassium hydroxide.
Because today""s more aerodynamic automobile designs allow less airflow through the xe2x80x9cunder the hoodxe2x80x9d space of the vehicles, the temperatures to which the surge tank is exposed have increased dramatically. These higher temperatures, coupled with the use of the newer engine coolants listed above can produce a much harsher environment for the surge tank. This harsh environment has resulted in greater discoloration and staining of the polymer from which the tank is made.
Automobile manufacturers in general and surge tank suppliers in particular were at first unconcerned about discoloration. However, as the sales volume of polyolefin surge tanks has increased, consumers have increasingly noticed and expressed concern about this discoloration. The consumers believe, falsely, that the polymer has somehow degraded. As engine coolant technology is evolving, new longer life (hybrid/organic) fluids are being developed for automobile engine radiator systems. Those new fluids will likely produce even greater degrees of discoloration on polyolefin parts such as surge tanks, and will further increase consumer concerns.
It should be noted that there are two types of discoloration of concern to the present invention:
(1) surface staining due to direct contact of the polymer with harsh chemicals such as automobile engine coolant; and
(2) discoloration caused by exposure to heat such as that experienced in the xe2x80x9cunder the hoodxe2x80x9d region of modern automobiles.
Traditionally, polyolefin manufacturers have included phenolic antioxidant additives such as pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) (Irganox(copyright) 1010, Ciba) and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate (Irganox(copyright) 1076, Ciba), to stabilize and protect the polymer. However, phenolic antioxidants permit the finished part to experience an unacceptable level of discoloration when subjected to higher temperatures as in today""s automobile engine and exposed to harsh chemicals such as the newer engine coolants, thereby leading to the above-mentioned consumer doubts about the integrity of products made from the polymers.
U.S. Pat. Nos. 5,844,029, 5,880,191 and 6,013,798 all describe thermoplastic resin compositions and methods of making them, wherein the compositions contain at least a saturated hydrocarbon amine oxide. Also disclosed are stabilizer compositions which contain at least a saturated hydrocarbon amine oxide and a second stabilizer. U.S. Pat. No. 5,844,029 describes the thermoplastic resin composition as containing only a saturated hydrocarbon amine oxide as a first component and cols. 5-8 list a wide variety of optionally included ingredients without any direction as to methods of choosing a particular ingredient from among those listed. No mention is made of using any other non-phenolic antioxidant in place of the saturated amine oxide. Further, no mention is made of using the thermoplastic resin to form articles which will be subjected to harsh temperature and chemical conditions.
U.S. Pat. No. 6,090,877 discloses low smoke forming, acid-neutralized polypropylene compositions comprising a polypropylene polymer and an acid scavenging amount of mono potassium citrate. The compositions of U.S. Pat. No. 6,090,877 are formulated so as to reduce clogging or obstruction of a screen pack that filters molten polymer during extrusion. As in U.S. Pat. No. 5,844,029, a long list of potential additives is given without any guidance as to selection criteria. No other acid scavengers are described. Also, no mention is made of using the compositions to form articles which will be exposed to harsh temperature and chemical conditions.
U.S. Pat. No. 6,146,574 describes a blend of polyolefin with the reaction of a functionalized polyolefin and a polyetheramine wherein the polyetheramine is grafted to the functionalized polyolefin. The polyetheramine helps the compositions retain dye or paint color. The compositions are especially useful for making fibers and may be used to make paintable automotive body parts.
U.S. Pat. No. 6,172,153 describes a polymer composition and fiber, film and fabric prepared therefrom. The compositions described in the examples all contain the conventional phenolic antioxidant Irganox(copyright) 1076.
Consequently, there is a need in the art for additive packages for polyolefins which will provide color stability in the resultant product without sacrificing polymer stability. The present invention provides such polyolefin additive packages, which will yield polymers having enhanced stain and discoloration resistance upon exposure to higher temperatures such as those experienced in automobile engines, and harsh chemicals such as long-life radiator coolants.
The present invention provides a polyolefin additive package comprising, about 0.025% to about 1% of at least one non-phenolic antioxidant and about 0.025% to about 1% of a hindered amine light stabilizer.
The present invention further provides an article comprising, a polyolefin and an additive package comprising 0.025% to 1% of at least one non-phenolic antioxidant and 0.025% to 1% of a hindered amine light stabilizer, wherein the article exhibits enhanced discoloration resistance upon exposure to high temperature and harsh chemical conditions.
The present invention also provides a method of enhancing discoloration resistance in an article, the method comprising, combining at least one olefin with an additive package to make a mixture, the additive package comprising about 0.025% to about 1% of at least one non-phenolic antioxidant and about 0.025% to about 1% of a hindered amine light stabilizer, polymerizing the mixture to make a polyolefin and forming the polyolefin into an article, wherein the article exhibits enhanced discoloration resistance upon exposure to high temperature and harsh chemical conditions.
The present invention yet further provides a method of making an automobile surge tank with enhanced discoloration resistance, the method comprising, combining propylene with an additive package to make a mixture, the additive package comprising about 0.025% to about 1% of at least one non-phenolic antioxidant and about 0.025% to about 1% of a hindered amine light stabilizer, polymerizing the mixture to make polypropylene, and forming the polypropylene into an automobile surge tank, wherein the automobile surge tank exhibits enhanced discoloration resistance upon exposure to high temperature and harsh chemical conditions.
These and other advantages and benefits of the present invention will be apparent from the Detailed Description of the Invention herein below.